1. Field of the Invention
This invention relates to a circuit useful, for example, as a pulse width adjustment circuit applicable to an optical communication system. The circuit can produce an output signal that has an adjustable pulse width or duty cycle to compensate for disturbances on the output signal caused by the communication system.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
A driver circuit or buffer is generally recognized as a circuit which can drive a transmitter. The transmitter will then forward the appropriate signal across the transmission path to a receiver. Depending on the transmitter and the transmission path, the driver can be subjected to various load constraints that can impart distortion or skewing of the output signal produced by the driver.
A popular form of driver is one that is used to drive a light source. The light source can be a photodiode or laser. A popular photodiode is a light-emitting diode (xe2x80x9cLEDxe2x80x9d), and a popular laser is a vertical-cavity surface-emitting laser (xe2x80x9cVCSELxe2x80x9d). VCSELs have distinct advantages over LEDs in that lasers can transfer data across the transmission path at much higher rates and for further distances than LEDs. With the advent of SONET and high-speed Ethernet systems, fiber-optic transmission paths have become increasingly pervasive as a communication system. Moreover, use of VCSEL diodes as the light source to transmit optical data is also becoming increasingly popular.
Depending on the technology used, the light-emitting diode can operate at differing voltages and at differing speeds. A high-speed application, however, is less tolerant to skews in the driver output signal sent to the diode. Moreover, lower operating voltages of the diode may require additional output drive on the edges of each pulse to ensure a pulse width ratio is maintained across differing operating voltages.
U.S. Pat. No. 5,856,753 (xe2x80x9cPatent ""753), herein incorporated by reference, describes a driver circuit which can maintain a given pulse width and duty cycle across different operating voltages. Thus, if the operating voltage should decrease, instead of the duty cycle also decreasing at a given measuring point, the pulse width or duty cycle can be adjusted to maintain the pre-existing duty cycle. That duty cycle can be, for example, 50%. In optical communication systems, it becomes necessary in most instances to maintain a given duty cycle near 50% even though the light-emitting diode can operate at differing voltages and can induce load distortion or skew onto the output signal produced from the driver and sent to the diode.
As described in Patent ""753, there are numerous ways in which to maintain a pulse width or duty cycle of the output signal sent from a driver into a load. The various methodologies described have certain disadvantages. It would be desirable to derive a driver circuit which can adjust the pulse width or duty cycle of an output signal produced by the driver circuit depending on the load or diode being used yet, however, the desired driver circuit produces an output signal that is symmetric on both the rising and falling edges of each pulse and does not require that the output signal be pulled up and down to the power supply rails. Accordingly, the desired driver circuit can adjust the pulse width or duty cycle of the output signal to compensate for disturbances in the load, and that the output signal purposely avoids voltage swings between the power supply rail and ground which would lessen the speed at which the output signal can transition. Thus, the desired output signal is one that has a low voltage swing and operates at high speed, readily used by VCSEL optical communication systems.
The problems outlined above are in large part solved by a high-speed, low-swing pulse width adjustment circuit and method. The circuit can adjust the pulse width or duty cycle of an output signal produced by the driver circuit. The driver circuit output signal does not transition between the power supply and ground voltages. Instead, the output signal transitions between a positive peak voltage and a negative peak voltage, where the reference voltage is preferably regulated at a voltage between the negative peak voltage and the positive peak voltage. Any change in the operating voltages of the load to which the output signal must be adjusted can be compensated for by correspondingly changing the reference voltage. Thus, if the output signal positive peak voltage decreases to accommodate a lower operating voltage of a load, the reference voltage can also decrease a proportionate amount so that the time at which each pulse extends upward from the reference voltage to the positive peak and back down to the reference voltage, will remain constant over dissimilar operating voltages (and dissimilar output signal peak voltages).
Adjusting the reference voltage in proportion to the output signal peak voltages, and any distortion or skewing affects of the load, ensures that the pulse width of the output signal can be regulated under any loading conditions. An input voltage to a source follower transistor will have a relatively small voltage swing. After chopping, this signal is further reduced. However, this signal can be amplified to achieve a desired swing for the driver. Additionally, regulating the reference voltage gives a desired pulse width and swing. This limited swing signal is particularly well-suited for driving a VCSEL diode for higher speed applications. If the reference voltage is properly regulated, a 50% duty cycle can be maintained for any distortion, skew, jitter, or operating voltage levels of a driver load device (e.g., a diode in an optical communication system). In certain optical communication systems, it is important to maintain a 50% duty cycle and, thus, the present driver can achieve that goal by simply regulating the reference voltage input to the driver circuit, where the output signal can maintain a symmetrical rise and fall transition without having to attempt adjusting the duty cycle by simply changing the fall transition and not the rising transition, as in certain prior art systems. A more robust pulse width and duty cycle regulator is, therefore, achieved by the improved driver circuit than conventional driver circuits.
According to one embodiment, a circuit is provided. The circuit comprises a comparator having an output and a pair of inputs. The pair of inputs are adapted to receive an output signal voltage produced from the circuit and a reference voltage forwarded to the circuit. The output of the comparator is coupled to a pull-down transistor that is connected to one of the pair of inputs of the comparator. The input of the comparator to which the pull-down transistor is connected is preferably the same input that receives the output signal.
According to another embodiment, a system is provided. The system is one that can adjust the pulse width or duty cycle of an output signal. The system includes a circuit for maintaining a reference voltage between the positive and negative voltage peaks of the output signal. A comparator is also provided and is coupled to compare a voltage of the output signal to the reference voltage. Depending, in part, on the slew rate and/or gain of the comparator, the comparator can fix the minimum voltage of the output signal to a voltage approximately equal to the reference voltage. The comparator is coupled in a feed-back arrangement so that the pulse width of the output signal varies in proportion to changes in the reference voltage. Along with the pulse width, a duty cycle of the output signal also varies in proportion to changes in the reference voltage. For example, an increase in the reference voltage will cause a corresponding decrease in the pulse width and duty cycle of the output signal. Conversely, a decrease in the reference voltage will cause a corresponding increase in the pulse width and duty cycle of the output signal. The slew rate and/or gain of the comparator is predetermined to preferably preclude a voltage of the output signal from being less than the reference voltage. The system can further include a pull-down transistor and an optical signal transmitter coupled to receive the output signal. Preferably, the transmitter is a VCSEL diode or LED.
According to yet another embodiment, a method is provided for regulating a duty cycle of an output signal. The method includes presenting the output signal into a comparator for comparing the output signal to a predetermined reference voltage and feeding back the results of the comparison to a pull-down transistor that chops the output signal between a periodic and symmetric positive peak voltage value and the reference voltage. The time at which the positive peak voltage value extends above the reference voltage and the ensuing duty cycle is directly proportional to the magnitude of the reference voltage.